1. Field of the Invention
This invention relates to a process for the hydroformylation of olefins.
More particularly, this invention relates to a hydroformylation process for separating and recovering triarylphosphine selectively from a liquid reaction product of the hydroformylation of olefins containing a Group VIII noble metal-triarylphosphine complex catalyst, free triarylphosphine, by-product triarylphosphine oxide, formed aldehyde and a reaction solvent, and recirculating it into a hydroformylation reaction zone.
2. Description of the Prior Art
It is well known that Group VIII noble metal-triarylphosphine complex catalyst, particularly Group VIII noble metal-triphenylphosphine complex catalysts, and more particularly, rhodium-triphenylphosphine complex catalyst are advantageous for industrial use as catalysts for the hydroformylation reaction in which an olefin is hydroformylated to form an aldehyde having one more carbon atom than the olefin. Group VIII noble metal-triarylphosphine complex catalysts, with which free triarylphosphine should preferably coexist, provide great advantages because of their high thermal stability, as follows:
(1) After the aldehyde formed is separated by distillation, gas blow stripping, or otherwise from a liquid product of the hydroformylation reaction containing the complex catalyst, the residue containing the complex catalyst can be recirculated into a hydroformylation reaction zone; and
(2) While the aldehyde formed is being separated from the reaction zone in gaseous form with the unreacted gas, by reactive distillation, for example, stripping with unreacted gas containing olefin, carbon monoxide and hydrogen, from the liquid product of hydroformylation containing the complex catalyst, or otherwise, the hydroformylation can be continued with the catalyst liquid containing the complex catalyst remaining in the reaction zone. In this case, the aldehyde condensation by-product having a high boiling point can be removed simultaneously (Japanese Laid-Open Patent Application No. 125103/1977).
The recirculation of Group VIII noble metal-triarylphosphine complex catalyst for reuse, or the continued use thereof in the reaction zone as described above is very advantageous, but nevertheless, presents the following problems which everybody engaged in the hydroformylation of olefins with such catalysts desires to solve:
The recirculation of a catalyst liquid containing a Group VIII noble metal-triarylphosphine complex catalyst, free triarylphosphine, etc. for reuse in an industrial process results in the collection in the catalyst liquid of a by-product formed mainly by secondary reactions of the aldehyde formed, and having a higher boiling point than the aldehyde, triarylphosphine oxide formed by oxidation of a part of triarylphosphine with oxygen dissolved in the solvent, present in the raw materials, or mixing into the process line during various steps of operation, or the like. In the event the catalyst liquid is continuously used in the reaction zone while the aldehyde formed is being separated therefrom by reactive distillation, etc., triarylphosphine oxide formed as described above, or the like collects in the catalyst liquid in the reaction zone, though the high-boiling aldehyde condensation by-product can be removed.
The accumulation of the high-boiling by-product, triarylphosphine oxide, etc., as described above leads to a corresponding increase in the volume of the catalyst liquid as a whole, and ultimately disables a reaction vessel having a fixed capacity to continue operation. Particularly, it is known that if triphenylphosphine is employed as triarylphosphine, the accumulation of triphenylphosphine oxide formed by oxidation of triphenylphosphine causes an undesirable increase in the formation of branched aldehyde and adversely affect the rate of the reaction (Japanese Laid-Open Patent Application No. 8207/1976).
The reuse of the catalyst liquid by recirculation or the continued use thereof over a long period of time leads to reduction in the activity of the catalyst. With the accumulation of reaction time, the catalyst deactivated by reaction inhibitors contained in minor quantities in the raw materials collects in the catalyst liquid, and the catalyst liquid has a gradually decreasing catalytic activity until it finally finds it difficult to maintain a desired rate of reaction.
These problems of the accumulation of the high-boiling by-product, triarylphosphine oxide and deactivated catalyst can be solved by removing a part of the recirculated catalyst liquid containing such substances uniformly, or a part of such catalyst liquid in the reaction zone continuously or intermittently from the reaction system, and replenishing the reaction zone with a corresponding quantity of an active Group VIII noble metal-triarylphosphine complex, or a rhodium salt as a source of such complex, and free triarylphosphine, and if required, a reaction solvent. This method makes it possible to maintain the concentration of each of the substances accumulated in the catalyst liquid being recirculated or remaining in the reaction zone at an equilibrium level which depends on the amount of the catalyst liquid removed and the amount of each such substance formed, thereby permitting stabilized operation of the reaction vessel for hydroformylation with a fixed concentration of such substances. The aforementioned equilibrium concentration is determined at an optimum level which is required on an industrial basis.
The catalyst liquid removed from the reaction system (hereinafter referred to as the spent catalyst liquid) contains useful and expensive triarylphosphine and Group VIII noble metal forming a complex therewith. It is, thus, very desirable from an industrial standpoint to recover and reuse such triarylphosphine and noble metal efficiently.
Various methods have hitherto been proposed for recovering Group VIII noble metals from the spent catalyst liquid. They include adsorption (Japanese Patent Publication No. 28273/1973, Japanese Laid-Open Patent Application No. 7114/1972, etc.), extraction with a strong acid (Japanese Patent Publication No. 43219/1971), and combustion (Japanese Laid-Open Patent Application No. 39690/1975).
On the other hand, only two methods are known for recovery of triarylphosphine, particularly triphenylphosphine, i.e., extraction with a strong mineral acid (Japanese Patent Publication No. 43219/1971), and extraction with formaldehyde-strong mineral acid (West German OLS 25 02 233). According to the former method, rhodium is extracted simultaneously with free triphenylphosphine, and the latter method also extracts free triphenylphosphine in aqueous layer with rhodium. In any event, as triphenylphosphine is extracted as a salt in the aqueous layer, it must be neutralized with an alkali for removal of counter ions such as sulfate ions (SO.sub.4.sup.--) and chloride ion (Cl.sup.-), so that it can be recirculated into the reaction zone for hydroformylation. As chlorine (Cl), sulfur (S), etc. are known to be poisonous to a rhodium catalyst (Japanese Laid-Open Patent Application Nos. 41805/1975 and 71610/1975), and can also cause corrosion to the reaction vessel, the triphenylphosphine recovered by these methods requires careful removal of those anions and purification prior to recirculation for reuse.
The inventors of this invention have made an extensive study as to any possible method for recovering triarylphosphine selectively from the spent catalyst liquid without involving any chloride or sulfate ion, or the like, that may be poisonous to the catalyst, or cause corrosion to the reaction vessel. As the result, the inventors have discovered that triarylphosphine, triarylphosphine oxide and Group VIII noble metal-triarylphospnine complex catalyst have different degrees of solubility in the high-boiling by-products of hydroformylation to the extent which is appropriate for the selective separation of triarylphosphine. This invention is based on such discovery.